The present invention generally relates to an optical pickup for use in an optical disk system, and more particularly, to a semiconductor laser device and a semiconductor laser chip used in the optical pickup.
FIG. 1 is a schematic view of an optical disk system that includes an optical pickup having a semiconductor laser device and using a three-beam tracking method. To simplify the drawing, a collimator lens, a beam splitter, and some other optical elements are not shown in FIG. 1. As a matter of convenience, a laser chip is shown enlarged without being drawn to scale.
Light emitted from a semiconductor laser chip 1, including a main beam 11 and sub-beams 12, is reflected from an information-recording surface including a track 7 of an information-recording medium 6 such as an optical disk (e.g., a compact disk or a magneto-optic disk). The reflected light of both the main beam 11 and the sub-beams 12, 12 passes through an objective lens 5 and then reflected by an inclined mirror 4 for diverting the light path. Then, the reflected light passes through a diffraction grating 3 and enters an end surface of the semiconductor laser chip 1 as returned light. The optical pickup is so constructed that the returned main beam returns to an emission point 9.
As shown in FIG. 2, in the optical pickup having the semiconductor laser chip 1, the optical axis of light emitted from the semiconductor laser chip 1 to the inclined mirror 4 is disposed approximately parallel (up to about 3xc2x0) to a tangential direction 8 of a track of the optical disk 6. In this case, the returned sub-beams 10 are incident on the end surface of the semiconductor laser chip 1 at points which are located at a distance of 70-80 xcexcm from the emission point 9 in a direction almost perpendicular (about 87xc2x0 to 90xc2x0) to a PN junction 2 in the semiconductor laser chip 1, as clearly shown in FIG. 3. In FIG. 3, reference numeral 15 represents a submount or stem on which the laser chip 1 is mounted, and reference numeral 16 represents an imaginary line connecting two returned sub-beams.
There are cases in which the returned sub-beams 10 are reflected from the end surface of the semiconductor laser chip 1, fed back to the optical system and adversely affect an S/N ratio.
In a conventional solution to the problem, the semiconductor laser chip is thinned or the PN junction is disposed at the center of the semiconductor laser chip. That is, by taking measures to prevent the returned sub-beams 10 from being incident on the end surface of the semiconductor laser chip, the returned sub-beams 10 are prevented from being fed back to the inside of the semiconductor laser chip.
Another conventional solution is to prevent the reflection of the returned sub-beams from the end surface of the semiconductor laser chip by reducing the reflectance of those portions of the end surface upon which the returned sub-beams are incident or applying a resin to such portions, instead of modifying the configuration of the semiconductor laser chip. These solutions, however, require a special processing of the semiconductor laser chip. Thus, these solutions cost more than the previously mentioned solutions of preventing the returned sub-beams from being incident on the end surface of the semiconductor laser chip.
The optical pickup 13 shown in FIG. 2 is arranged such that the optical axis of light emitted from the semiconductor laser chip 1 to the inclined mirror 4 is approximately parallel (up to about 3xc2x0) to the tangential direction 8 of the information-recording track of the optical disk 6, as described above. In this case, in reading and writing information from and to an outermost track 14 of the optical disk 6 loaded, the optical pickup 13 protrudes largely outward from the optical disk 6.
Thus, it is an object of the present invention to provide an optical pickup which hardly protrudes from an optical disk loaded to thereby make an entire optical disk system compact while cutting off a noise caused by returned sub-beams, and also to provide a semiconductor laser chip and a semiconductor laser device that realize such an optical pickup at low costs.
According to an aspect of the invention, there is provided a semiconductor laser chip having an emission point on a PN junction thereof, wherein said emission point is located at a distance of 80 xcexcm or less along the PN junction from a lateral side surface of the semiconductor laser chip.
The semiconductor laser chip may have a light emission end surface of a shape of a parallelogram. The parallelogram may have right angles or oblique angles at its vertexes.
The semiconductor laser chip may have a width of 160 xcexcm or less, wherein the laser chip width is defined between opposite lateral side surfaces of the semiconductor laser chip. In this case, the emission point is located within 80 xcexcm of both lateral side surfaces.
An optical pickup according to another aspect of the present invention has the semiconductor laser chip as described above. The optical pickup reads information from an information-recording medium by irradiating the information-recording medium with light emitted from the semiconductor laser chip. When light is emitted from the semiconductor laser chip, there take place a plurality of returned beams from the information-recording medium to a plane including an end surface of the semiconductor laser chip, and an imaginary line connecting points of incidence of the returned beams on the plane forms an angle of less than 90xc2x0 with the PN junction of the semiconductor laser chip.
The plurality of returned beams may comprise a returned main beam and one or more returned sub-beams. The one or more returned sub-beams are not incident on the end surface of the semiconductor laser chip.
The present invention also provides a semiconductor laser device for use as a light source of an optical pickup detecting a track signal by a method using sub-beams. The semiconductor laser device includes a semiconductor laser chip having an emission point that is located at a distance of 80 xcexcm or less from a lateral side surface of the semiconductor laser chip.
An optical pickup according to a further aspect of the present invention has a mirror which diverts a light path toward an information-recording surface of an optical disk placed in position, and a grating which generates one or more sub-beams. The grating is disposed in such a way that an angle xcex8 of 5xc2x0-90xc2x0 is formed between the information-recording surface and a direction in which the grating is oriented.
The present invention prevents returned sub-beams from being incident on the end surface of the semiconductor laser chip by locating the emission point of the semiconductor laser chip within 80 xcexcm of one lateral side surface thereof even though an optical axis of light travelling from the semiconductor laser chip to the diverting mirror is inclined with respect to the tangential direction of a track of the optical disk. Thus, the present invention avoids the problem that the returned sub-beams are reflected from the end surface of the semiconductor laser chip and fed back to the optical system to adversely affect the S/N ratio. At the same time, the present invention realizes size reduction of an entire optical disk system.
The position in a horizontal direction with respect to the PN junction of the emission point of the semiconductor laser chip can be easily achieved by adjusting a marking-off position during a chip-dividing step of the semiconductor laser chip production process, while the position in a vertical direction with respect to the PN junction of the emission point can be easily achieved by adjusting an amount of crystal growth during the semiconductor laser chip production process. Thus it is unnecessary to carry out an additional process step such as processing of the end surface of the semiconductor laser chip or application of resin to the end surface of the semiconductor laser chip.
Owing to the construction described above, even without additional processing to the laser chip, the present invention prevents deterioration in performance of the semiconductor laser device due to reflection of returned sub-beams from the end surface of the semiconductor laser chip, which has been the problem in manufacturing a compact end product. The present invention achieves the simplification of the manufacturing process and the improvement of product quality.